1. Field of the Invention
The present invention is directed to the field of automatic surface inspection systems and more particularly to the inspection of highly reflective surfaces such as silicon wafers, for minute flaws.
2. Description of the Prior Art
Semiconductor manufacturers utilize silicon wafer discs for the base substrate in the manufacture of solid state electronic components, such as transistors and integrated circuits.
The occurrence of various types of flaws, such as dust, dirt, crystal imperfections, scratches, haze and pits, down to the low micron size are detrimental to the component fabrication process and adversely affect the yield of individual components in production.
At present, a manual, visual inspection technique is used by most manufacturers of the silicon wafers and fabricators of integrated circuits. The manual technique employs an intense light source to illuminate the wafer surface. A human inspector adjusts the wafer to an appropriate angle of light reflection and visually observes the wafer surface for several seconds to determine the surface quality of the wafer. While the manual technique is adequate for detecting dirt and dust particles and scratches down to approximately 5 microns, imperfections of lesser size go undetected.
A description of a technique utilizing a scanning laser beam to inspect silicon wafers is found in an article entitled "A Laser Scan Technique for Electronic Material Surface Evaluation" by D. R. Oswald and D. F. Monroe, published in the Journal of Electronic Materials, Vol. 3, No. 1, 1974, pages 225-241. In the described device, a beam of electromagnetic radiation from a three milliwatt laser is first expanded from its original diameter to a larger diameter and is directed unto a torsional oscillating mirror that directs the expanded beam at 90.degree. into a lens. The rotational axis of the mirror lies in the front focal plane of the lens and intersects its optic axis. The wafer surface to be examined is positioned on the other side of the lens, one focal length from it. The focused beam is normally incident to the surface of the wafer and is line-scanned over the surface as the wafer is transversely transported past the scanning line.
In the absence of defects, all the light is said to be normally reflected from the surface and follows the incident path in reverse. When the focused spot strikes the wafer surface and encounters a defect, light is said to be scattered by that defect so that the regional space immediately surrounding the main return beam contains the scattered light energy. An apertured mirror is placed in the beam path between the laser generator and the oscillating mirror so as to allow transmission of the expanded beam from the generator to the oscillating mirror and provide a reflecting surface for the received scattered light.
The scattered light reflected from the apertured mirror is focused by a receiving lens onto a signal detector. The variation in the scattered light received by the signal detector is used to indicate the presence of defects on the surface of the wafer.
A memory oscilliscope and defect counter are employed to show location and accumulate the occurrence of defects, respectively.
The prior art device is described as successfully detecting and counting defects greater than 8 microns in diameter and some defects down to 1 micron in size, providing they have proper light scattering characteristics.